1. Field of the Invention
This invention relates to testing equipment for integrated circuits, and more particularly, to test breakout boards and their layout.
2. Description of the Related Art
There are many ways to test electronic circuits while the circuit is operating. One method, in particular, is to place the device-under-test (DUT) in a known good operational environment. While in operation, the DUT signals are monitored by test equipment, such as a logic analyzer or oscilloscope.
A common method of monitoring the DUT signals employs a printed circuit board (PCB) known as a breakout board. The breakout board typically includes a device socket, which allows the DUT to be inserted and removed without the need for soldering. The breakout board also provides a means for intercepting the DUT signals and routing them to a test connector. The breakout board is physically located between the DUT and the system board. Therefore, the breakout board also provides a means to connect to the system board in the same way that the DUT would connect to the system board if the breakout board were removed. This is sometimes accomplished by mounting a socket on the system board and a socket adapter on the breakout board.
The breakout board typically has connection areas called pads on the surface of the top layer for soldering a DUT test socket. The pads are typically arranged in a pattern that matches the DUT connection pattern, which is known as a footprint. This footprint is typically duplicated on the bottom PCB layer directly opposite the top layer.
The top layer pads and the bottom layer pads are typically connected together by connections called vias, which are described further below. Small wires called traces connect the pads to power, ground and each other. Since there are often more traces needed than can be manufactured on the surface layers alone, traces are placed on various internal layers. To connect the various internal layer traces to each other and to the surface layers, metal plated connections are used. These connections are called vias. Two types of vias are commonly used: Plated through-hole vias and blind vias.
Plated through-hole vias are holes drilled completely through all layers of the PCB and perpendicular to the surface. The holes are then plated with a conducting metal, such as copper or gold. Through-hole vias can therefore connect signals on any layer to any other layer. Blind vias are holes drilled from the surface through some of the internal layers and then plated with a conducting metal. Therefore, blind vias can connect signals on the surface layer to any internal layers through which it passes.
As shown in FIG. 1, on the breakout board PCB 10, through-hole vias 11 connect the top and bottom contact pads together. The signal traces 16 on various layers connect to the vias at right angles and are routed to the through-hole vias 21 below the test connector 14. The test connector 14 typically includes contact pins 15, which extend down through the through-hole vias 21, where they are soldered to the bottom surface of the PCB. Any test equipment connected to the connector may now receive the DUT signals.
This design works at some frequencies. However, as device clock and signal frequencies increase, circuit board layout becomes a more critical step in producing a fully operational system. Since the PCB traces are transmission lines, their lengths, widths and routing become significant with an increase in frequency in terms of signal quality and strength. In the breakout board design of FIG. 1, the internal layer trace connections to the vias act like transmission line stubs 30. These stubs can create an impedance mismatch at certain frequencies, which can adversely affect the signals. The signal degradation may cause incorrect test readings or complete failure of the DUT in the system during testing. Therefore, it is desirable to provide a better impedance match at the frequencies of interest.
The problem outlined above may in large part be solved by a breakout board design using blind vias to eliminates stubs on critical timing signal paths.
In one embodiment, a test breakout board comprising a printed circuit board (PCB), which may be implemented in multiple layers, includes a first set of contact pads on one side of the PCB and a second set of contact pads, which are directly opposite the first set of contact pads, on the other side of the PCB. Each pad in the first set of contact pads is connected to a corresponding pad in the second set of contact pads through a pair of blind vias, a pair of signal traces and a throughhole via. Each of the through-hole vias is connected to a corresponding contact pin in a test connector. The test connector is mounted on the top contact of the PCB and provides an interface to test equipment such as a logic analyzer or an oscilloscope. The breakout board may also include a test socket for holding a device-under-test (DUT), such as a microprocessor or an application specific integrated circuit (ASIC) chip. The integrated circuit chip is enclosed in a device package to protect the die. The test socket includes contacts, which are soldered to the first set of contact pads on the top surface of the breakout board. Additionally, the breakout board may also include an electrical interface adapter, which mechanically resembles the DUT. The interface adapter is soldered to the second set of contact pads on the bottom surface of the breakout board and provides the interface between the test breakout board and a system motherboard.
In another embodiment, a test breakout board is connected to a system motherboard. The system motherboard includes an additional test socket, which can hold a DUT, or in this case, a test breakout board electrical interface adapter. The electrical interface adapter mechanically fits into the additional test socket. The contacts on the adapter make contact with a set of metallic contacts in the socket, thereby providing a signal path from the system motherboard, through the breakout board, to the DUT. A test connector on the test breakout board allows test equipment, such as a logic analyzer or an oscilloscope, to be connected to the test breakout board.
The signal path from each of the contact pads through each blind via, through a signal trace, a through-hole via, a second signal trace and a second blind via, to each second contact pad may advantageously provide an impedance matched signal path from a system motherboard to a DUT and a test connector. Thus, the test breakout board may facilitate proper testing at high frequencies.